1. Field of the Invention
The present invention generally relates to hydrogen sensors and more particularly to such sensors being used to detect hazardous hydrogen buildup conditions in and around a fuel cell.
2. Description of the Prior Art
As fuel cell applications become more widespread, the need for hazardous hydrogen concentration sensing in and around such fuel cells becomes necessary. Specifically, a 4% hydrogen concentration in air constitutes an explosive mixture. Accordingly, a hydrogen sensor is needed to monitor fuel cell assemblies for hydrogen fuel leaks.
Numerous methods to detect hydrogen exist. The methods range from expensive, high sensitivity instruments (gas chromatographs and mass spectrometers capable of detection of sub-parts per million) to low-cost, simple methods such as a pair of thermocouples operated in differential mode with one of the thermocouples coated with a catalyst. None of these methods are suitable for use in or around a solid oxide fuel cell stack. Laboratory instruments such as mass spectrometers are not capable of in-situ operation and cannot operate in a continuous, unattended manner. Thermocouples can operate in-situ, but tend to drift at solid oxide fuel cell operating temperatures of 900xc2x0 C. Furthermore, the catalyst coatings used in the hydrogen detectors can be poisoned by even small amounts of CO or SO2 gases which may be present in some fuel cell fuels. Plus, since thermocouples are electrical devices, they are not intrinsically safe, insofar as they constitute an arc and spark hazard and an impractical ground fault protector would be needed to use them in environments where an explosion hazard exists.
What is ultimately needed for hydrogen detection is a low-cost, low-drift in-situ sensor that can function for thousands of hours of continuous use in a high temperature environment, and which is capable of repeatable measurement of hydrogen concentration in the 1 to 10% range. It is also very desirable that the hydrogen sensor be intrinsically safe.
The basic configuration of the invention consists of an optical fiber connected at one end to a signal conditioning and processing unit. Light from the signal conditioner is transmitted through the fiber to the opposite (terminal) end of the fiber, which defines the hydrogen sensor location. The terminal end is coated with palladium (Pd) metal which acts as a reflector to return light to the input end. The optical constants (refractive index and absorption coefficient) of Pd change when it is exposed to hydrogen. The Pd metal also swells as it soaks up hydrogen much, like a sponge soaks up water.
The changes in the optical constants of Pd, which consequently affect the intensity and phase of light reflected back into the fiber optic cable, are utilized in some embodiments of the present invention. Specifically, the change in refractive index of the glass fiber, which occurs as a result of the stress induced by the swelling Pd, is employed. When the optical constants of Pd change, the optical phase shift of the reflection changes; similarly, when the stress in the fiber changes (due to Pd swelling), the fiber""s refractive index changes in proportion to the stress which, in turn, alters the phase of an optical signal traveling through the stressed portion of the fiber. Ultimately, these aforementioned changes can be detected by measurement of the resultant phase shift of light reflected back to the signal processor.
A two-beam interferometer can be used to detect the phase shift with the interference occurring at the photodetector in the signal processor unit. The interferometer compares the light entering the fiber with the light reflected by the Pd coating. Using electronic methods and components well known to those skilled in the art, the magnitude of the phase shift is converted to a signal voltage that changes in proportion to changes in the phase shift. The sensitivity and dynamic range are more than adequate to detect changes in hydrogen concentration of 4%, and provide sufficient margin to set an alarm threshold.
In view of the foregoing it will be seen that one aspect of the present invention is to detect a hazardous hydrogen concentration in any system which employs hydrogen gas, and especially in and around a fuel cell.
Another aspect is to provide an intrinsically safe hydrogen sensor, with particular utility for detecting fuel cell leaks.
Yet another aspect is to provide a multiplexed hydrogen leak sensor system, again with particular utility for detecting hydrogen in a plurality of fuel cells.
Accordingly, a fiber optic hydrogen sensor, for sensing the overall hydrogen concentration in a system which utilizes a gas mixture at least partially composed of hydrogen gas is disclosed. The sensor comprises a fiber optic cable, with a reflective, coated end and with a specific type of light passing through it, and means for detecting changes in the intensity and/or phase of the reflected light which is representative of the overall hydrogen concentration in the part of the system which is being monitored. Additionally, the sensor may use a palladium coating for the fiber optic cable, a photodector, an interferometer, a light coupler, a 2xc3x972 light splitter, an alarm and/or automated control valves. Ideally, the sensor may be used on one or more fuel cells (or other, repeating units in a system). Finally, where appropriate, the sensor may also utilize a set point in order to compare the measured signal against a known standard of hydrogen concentration.
These and other aspects of the present invention will be more fully understood upon a review of the following description of the preferred embodiment when considered in conjunction with the drawings.